We teachers do a lot to make sure we are fair and consistent in our marking. We have meetings about it, we second mark or moderate (this is where we mark behind someone else, and then see whether the mark we would give a paper meets up with the original mark), and we talk about it. A lot. The students trust us to be fair, and we do everything in our power to be. But it is still difficult. Sometimes a paper is *almost* there, but not quite. We see the hard work that goes into these papers; we know how stressed-out our students get. But, in the end, it's what's on the paper that counts.
In that way, it's like anything that I write--fiction or non-fiction. I can't sit on my reader's shoulder and explain what I meant here, or what I thought they'd understand there. All there is, is what's on the paper in front of them. There's a contract between reader and writer, and there's a contract between teacher and student.
You hold up your end, I'll hold up mine.
Here, have a photo of Queen Elizabeth I, from the inside of Hatfield House.